Resonant pumped short cavity fiber laser

ABSTRACT

A fiber laser or fiber amplifier uses resonant pumping of the gain medium by providing a pump resonator that establishes a resonator cavity at the pump wavelength which includes the pumped gain medium. The pump resonator may be of a distributed feedback (DFB) or a distributed Bragg reflector (DBR) type construction, and may be combined with signal reflection apparatus of either DFB or DBR type construction that provides oscillation of the desired laser output wavelength. If used without a signal reflection apparatus, the invention may be operated as a resonant pumped fiber amplifier. Resonant pumped lasers may be arranged in series to provide a laser apparatus with a selectable output wavelength. The different laser stages each provide resonance for a different pump wavelength, and each provide resonance for a different signal wavelength. Thus, a particular pump wavelength input to the series arrangement only resonates in one of the laser stages, and therefore only pumps the gain medium of that stage, allowing it to output its unique signal wavelength. The output signals from each laser stage may be coupled out of the series arrangement with wavelength selective optical couplers, each associated with a different laser stage.

FIELD OF THE INVENTION

This invention relates to the field of optical signal processing and,more particularly, to the generation of a low-noise, single frequencylaser output.

BACKGROUND OF THE INVENTION

In optical communications, it is desirable to use a communicationwavelength in the wavelength range of approximately 1.5 micrometers(μm). This is because 1.5 μm is the lowest loss wavelength ofconventional single-mode glass fibers. As the number of desired channelsand demand for high video quality increases, the corresponding demandson an optical transmission source also increase. Among these is thedemand for a low-noise, single-frequency 1.5 μm laser source. Fiberlaser sources have been created which use an erbium (Er) doped opticalfiber as a gain medium. However, such fiber lasers typically suffer froma tradeoff between gain and power, and mode discrimination.

It is commonly understood that, within a fiber laser, longitudinal modespacing is inversely proportional to the length of the laser cavity.That is, for a longer resonator cavity, the frequency spacing betweenadjacent resonance frequencies is smaller. As a result, morelongitudinal modes are within the (necessarily finite) width of the gainspectrum, and mode discrimination is poor. Even if the effective gainspectrum of the fiber laser is narrowed through the use of narrow bandreflectors, such as Bragg gratings written directly into the core of thefiber, many modes may exist with that effective gain bandwidth. For thedesigner of a single-frequency laser, this causes several difficulties.Firstly, the wide spectrum emitted from the laser interferes with otherlaser sources transmitted at different wavelengths in wavelengthdivision multiplexed systems. Secondly, dispersion in the wide spectrumresults in pulse spreading. Finally, mode competition betweenlongitudinal modes causes amplitude instability.

By making the laser cavity shorter, the mode spacing is increased and,if the cavity is short enough, the gain spectrum may be limited to asingle resonance frequency. However, the length of the gain medium (i.e.the doped fiber surrounding the cavity) also affects the optical gain ofthe laser. With a relatively short length of fiber, the gain of thelaser is low since there is less distance across which the pump energymay be absorbed within the fiber.

It would be desirable to have an optical source that has an output powersignificantly higher than previously available, and which the benefitsof a short fiber gain medium.

SUMMARY OF THE INVENTION

The present invention provides a single-frequency laser source in whicha high output power is achieved by developing a resonant condition forthe pump energy wavelengths within the laser cavity. In general, thelaser source has an optical fiber gain medium and a signal reflectionapparatus that reflects light at the signal wavelength within the gainmedium, so as to establish a resonance condition at the signalwavelength. The signal reflection apparatus is substantially entirelyreflective when it comes to optical energy in the cavity travelingtoward an input side of the cavity, and partially reflective to opticalenergy in the cavity traveling toward an output side of the cavity. Thispartial reflectivity provides the desired output coupling for the laser.The gain medium is pumped by a pump source that couples light at anappropriate pumping wavelength into the laser cavity. Once coupled intothe laser cavity, the pump energy is retained within the cavity using apump resonator that reflects light back and forth within a resonantcavity that includes the gain medium. This maximizes the pumpingefficiency, particularly for the short fiber laser, by retaining thepump energy within the gain medium.

The signal reflection apparatus may take different forms, as may thepump resonator. For example, the signal reflection apparatus may have adistributed feedback (DFB) type configuration in which a periodicgrating is built into a pumped part of the gain medium. As known in theart, a DFB grating has a shift in its structure that results in a1/4-wavelength phase shift in light at the chosen wavelength (e.g., thesignal wavelength), thus giving the desired resonance condition.Alternatively, the signal reflection apparatus may also have adistributed Bragg reflector (DBR) type configuration, in which an inputperiodic grating is located relatively close to an input side of thegain medium and an output periodic grating is located relatively closeto an output side of the gain medium. The input grating is highlyreflective to keep the optical energy at the signal wavelength withinthe cavity, while the output grating is partially reflective to allowoutput coupling of the laser energy.

The configuration described above for the signal reflection apparatusmay be combined with different configurations for the pump resonator.The pump resonator may be a DFB type structure that is reflective at thepump wavelength and that is built into a part of the gain mediumoccupied by a DFB type signal reflection apparatus grating. The pumpresonator may also be a DBR type structure having two highly reflectiveperiodic gratings, one on either side of the signal reflectionapparatus, that reflect optical energy at the pump wavelength back andforth within the gain medium. The DBR type pump resonator may be usedwith either a DFB type or a DBR type signal reflection apparatus. Ineither circumstance, the gratings of the pump resonator surround thegrating or gratings of the signal reflection apparatus, and provide"resonant pumping" of the gain medium across the area occupied by thesignal reflection apparatus. Also, when a DBR type signal reflectionapparatus is used with a DBR type pump resonator, a Q-switch may also beused to generate a high-intensity, short-duration pulse.

In one variation of the invention, a DBR-type arrangement of pumpgratings may be used to construct a short fiber amplifier similar to theshort fiber lasers discussed above. However, rather than using a signalreflection apparatus which develops the desired signal wavelength suchthat it originates in the gain medium, only the pump resonator is used.The signal to be amplified is then introduced into the pumped gainmedium, where it is amplified before being output. The resonant pumpingof the gain medium maintains a desired level of population inversion,and allows the signal passing through the gain medium to be amplified bystimulated emission. Also, with the use of only a single, broadbandsignal reflector, an amplified spontaneous emission (ASE) source may beformed that provides a broadband optical output.

In one embodiment of the invention, a plurality of resonant-pumpedlasers are arranged in series and allow an optical signal to begenerated at any of a plurality of different signal wavelengths. Each ofthe laser cavities may have a different combination of DFB and DBR typesignal reflection apparatus and pump resonator. For each laser, a DFBtype signal reflection apparatus may be combined with either a DFB or aDBR type pump resonator, or a DBR type signal reflection apparatus maybe combined with a DBR type pump resonator. Each of the lasers in serieshas a signal reflection apparatus constructed to oscillate at a signalwavelength different than the signal wavelengths of the other lasers.Furthermore, each of the lasers has a pump resonator constructed tooscillate at a pump wavelength different than the pump wavelengths ofthe other lasers. Thus, when a first pump wavelength is directed intothe series arrangement of lasers, it resonates only in a first lasercavity that has its pump resonator constructed to oscillate at the firstpump wavelength. This results in the gain medium of the first lasercavity being resonant pumped, while the other laser cavities achieve nosuch pumping. The pumping of the first laser cavity causes thegeneration of signal energy within the first cavity, which resonates ata first desired signal wavelength, being reflected within the cavity bythe signal reflection apparatus and eventually coupled out of the firstlaser cavity. When a different pump wavelength is introduced into theseries laser arrangement, it oscillates in the pump resonator of adifferent one of the laser cavities, causing that laser cavity to beresonant pumped, and producing a laser output at a second desired signalwavelength. Thus, the wavelength of the laser output is selectable byselecting the wavelength of the input pump energy.

In one variation of the series laser embodiment described above, awavelength selective element is used at the output of any of the lasers.The wavelength selective element at the output of a given laser coupleslight at the signal wavelength of that laser (i.e. the wavelength atwhich the signal reflection apparatus of that laser is designed toresonate) to a first output port, while directing other wavelengthsthrough to any subsequent lasers of the series arrangement. This avoidsany losses associated with the output from a given laser passing throughsubsequent optical components.

In another embodiment of the invention, the pump wavelength isstabilized to prevent reduced efficiency in a resonant pumped laser dueto wavelength drift of the pump source. A first variation of thisembodiment uses pump light that has a time-varying wavelength due tomodulation of a electrical input signal to the optical signal generator,creating two small frequency modulation (FM) side band frequencies.Light reflected from the laser cavity is then used to generate afeedback. The reflected light has an intensity which is minimum (and thetransmitted light a maximum) when the light directed to the cavity has awavelength that satisfies the peak resonant condition of the cavity.When this peak resonance condition is satisfied, a relative time-varyingintensity phase between the reflected optical energy and the pump sourceoptical energy is also a minimum. However, when the wavelength of thepump source drifts away from the wavelength for which the pump resonatorof the laser achieves peak resonance, a phase shift develops between thetwo FM sidebands resulting in an FM-to-amplitude modulation (AM)conversion. The relative strength of this AM signal is a measure of howfar the pump laser is from its resonance condition. Therefore, thissignal may be used to control the wavelength of the pump laser.

The reflected pump energy is coupled to a photodetector that converts itto an electrical feedback signal having a magnitude proportional to theintensity of the reflected optical signal. To control the wavelength ofthe pump laser, this electrical signal may then, for example, be inputto an electrical mixer that combines it with an AC component of theelectrical input signal that drives the optical signal generator. Themixing of these two AC signals results in DC offset signal that iscombined with the electrical input signal to increase or reduce thedriving current to the optical signal generator, and thereby adjust itswavelength to maintain resonance. If the pump signal has drifted to alonger wavelength, the phase shift between the reflected optical signaland the pump source optical signal causes the DC offset to increase thedriving current of the optical signal generator and therefore shortenthe pump signal wavelength. Conversely, if the pump signal has driftedto a shorter wavelength, the phase shift between the reflected opticalsignal and the pump source signal causes the DC offset to decrease thedriving current of the optical signal generator, thereby lengthening thepump signal wavelength.

Preferably, the current driven component of the optical signal generatoris a semiconductor diode which is supplied with a DC current componentfrom a DC electrical power source and an AC current from an AC signalsource. Optionally, an optical filter may be used to block wavelengthsin the reflected optical energy which are outside the desired wavelengthrange. Also, the optical signal from the optical signal generator may bepassed through an optical isolator prior to its reaching the lasercavity, to avoid reflected signals from returning directly to the signalgenerator.

In another wavelength-stabilized variation of the invention, the pumpwavelength is stabilized using optical feedback from the laser cavity.The optical pump signal is transmitted to the laser cavity via anoptical circulator which receives the optical signal from a signalgenerator along a first optical path. While the majority of the opticalpump energy is contained within the laser cavity by the pump resonator,a small amount of this optical energy is also transmitted past the lasercavity. The amount of optical energy transmitted past the cavity ismaximum when the wavelength of the light from the pump signal generatorsatisfies the peak resonance condition of the pump apparatus (i.e.exactly matches the wavelength for which the grating or gratings of thepump resonator were designed). The transmitted optical energy is coupledback to the optical circulator, which directs it to the first opticalpath in a propagation direction opposite to the propagation direction ofthe signal from the signal generator. Thus, the transmitted opticalsignal is returned to the signal generator as an optical feedbacksignal. The signal generator locks onto the frequency of the feedbacksignal, and resonates at the frequency of the feedback signal. Thus,satisfying the peak resonant condition of the laser cavity provides thestrongest feedback signal, and the strongest lock on the wavelength ofthe pump signal output by the optical signal generator.

The optical signal generator of the optical feedback embodiment ispreferably a fiber laser that includes a doped optical fiber pumped witha semiconductor laser diode. Coupling of the transmitted optical energyback to the optical circulator is preferably accomplished using awavelength selective optical coupler, such as a wavelength divisionmultiplexer (WDM). The WDM couples only wavelengths in the range of thepump wavelength to be directed back to the optical circulator, whileother wavelengths (such as the signal wavelength) are directed to asignal output port. An optical filter may also be used between thedirectional coupler and the optical circulator which blocks opticalenergy outside of the wavelength range of the pump wavelength.Optionally, the magnitude of the transmitted optical energy may also beattenuated with an optical attenuator in an optical path between thedirectional coupler and circulator.

In another variation of the invention, multiple laser cavities can bepumped using a single pumping source. The laser cavities may beconstructed to each have a different output signal wavelength and apredetermined distribution of output powers. In the preferredembodiment, the output powers of all the different lasers are equal. Thelasers are all fed from a single pump source using a multiple branchoptical coupler. The optical coupler may be a fused fiber opticalcoupler, and receives the optical pump energy from the pump source,distributing it to each of the laser cavities. Each of the laserresonators has a gain medium which may be pumped by the optical energyat the wavelength of the energy from the pump source. However, eachresonates at a different signal wavelength. Thus, each of the lasersgets an equal distribution of pump power, but outputs a differentwavelength. In one embodiment the lasers are resonant pumped using pumpgratings on each which retain the pump energy within the gain medium.Furthermore, the stability of the pumping source may be stabilized usingeither electronic or optical feedback.

In a construction similar to that described above, a multiple branchdirectional coupler may also be used to couple a plurality of differentlaser outputs, each having a different wavelength, into a signal opticalwaveguide. The laser cavities are each located in a different gainmedium, each of which is coupled into a single waveguide by thedirectional coupler. The coupler may be a fused fiber coupler, and pumpenergy from a single pump source is coupled into the laser cavities fromthe single waveguide through the directional coupler. The pump sourcemay be a semiconductor laser diode. A wavelength selective coupler(preferably a WDM) is used to direct optical energy from the pump sourceinto the waveguide, while preventing energy outside the wavelength rangeof the pump wavelength from reaching the pump source. In one embodimentthe lasers are resonant pumped using pump gratings on each which retainthe pump energy within the gain medium. Furthermore, the stability ofthe pumping source may be stabilized using either electronic or opticalfeedback.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a schematic view of a resonant pumped fiber laser accordingto the present invention that uses a signal energy reflector and a pumpenergy reflector each of which are of a distributed feedback (DFB) typeconstruction.

FIG. 1B is a schematic view of a resonant pumped fiber laser accordingto the present invention that uses a signal energy reflector having aDFB type construction and a pump energy reflector having a distributedBragg reflector (DBR) type construction.

FIG. 1C is a schematic view of a resonant pumped fiber laser accordingto the present invention that uses a signal energy reflector and a pumpenergy reflector each of which has a DBR type construction.

FIG. 1D is a schematic view of a resonant pumped fiber laser similar tothat show in FIG. 1C, but which is Q-switched.

FIG. 1E is a schematic view of a resonant pumped fiber amplifieraccording to the present invention.

FIG. 1F is a schematic view of a resonant pumped amplified spontaneousemission source according to the present invention.

FIG. 2A is a schematic view of a resonant pumped laser apparatusaccording to the present invention with a selectable output wavelength,the apparatus having two resonant cavities in series, each of which hasa construction as shown in FIG. 1A.

FIG. 2B is a schematic view of a resonant pumped laser apparatusaccording to the present invention with a selectable output wavelength,the apparatus having two resonant cavities in series, each of which hasa construction as shown in FIG. 1B.

FIG. 2C is a schematic view of a resonant pumped laser apparatusaccording to the present invention with a selectable output wavelength,the apparatus having two resonant cavities in series, each of which hasa construction as shown in FIG. 1C.

FIG. 3A is a schematic view of a resonant pumped laser similar to thatshown in FIG. 2B, but which also includes a wavelength selective outputcoupler for resonant cavities of the laser to allow low-loss outputcoupling of the laser outputs.

FIG. 3B is a schematic view of a resonant pumped laser similar to thatshown in FIG. 2C, but which also includes a wavelength selective outputcoupler for resonant cavities of the laser to allow low-loss outputcoupling of the laser outputs.

FIG. 4A is a schematic view of a resonant pumped laser that useselectrical feedback to stabilize the wavelength output of the opticalpumping source.

FIG. 4B is a schematic view of a resonant pumped laser that uses opticalfeedback to stabilize the wavelength output of the optical pumpingsource.

FIG. 5A is a schematic view of a multiple output laser that has multiplelaser resonators pumped by a single optical pumping source.

FIG. 5B is a schematic view of a laser source which has multiple laserresonators pumped by a single optical pumping source, and which combinesthe discrete wavelength outputs of the different resonators on a singleoptical waveguide.

FIG. 6A is a schematic view of a multiple output laser that has multiplelaser resonators that are resonant pumped and use a single opticalpumping source that is wavelength stabilized using electronic feedback.

FIG. 6B is a schematic view of a laser source having an output which isa combination of a number of discrete wavelengths, the laser sourcehaving multiple laser resonators that are resonant pumped and use asingle optical pumping source that is wavelength stabilized usingelectronic feedback.

FIG. 7A is a schematic view of a multiple output laser that has multiplelaser resonators that are resonant pumped and use a single opticalpumping source that is wavelength stabilized using optical feedback.

FIG. 7B is a schematic view of a laser source having an output which isa combination of a number of discrete wavelengths, the laser sourcehaving multiple laser resonators that are resonant pumped and use asingle optical pumping source that is wavelength stabilized usingoptical feedback.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Shown in FIG. 1A is a schematic depiction of a short fiber laser 11consisting of doped optical fiber 10 in which are etched two gratings.The gratings consist of a first grating (the "signal" grating) 12 and asecond grating (the "pump" grating) 14. Each of the gratings is awavelength-selective reflector etched into the cladding layer of thefiber 10, and each therefore reflects light at a predeterminedwavelength. The arrangement of FIG. 1A is similar to a "distributedfeedback" (DFB) laser structure, which is known in the art. In a DFBlaser, a periodic grating is built right into a pumped part of the gainregion. The grating is a periodic variation of refractive index alongthe gain medium that causes reflection at a desired signal wavelength.As is know in the art, the periodic variation includes a shift atroughly its center, which results in a phase shift of one quarter of thepredetermined wavelength. Thus, the structure of FIG. 1A uses a"λ/4-shifted" grating structure, as is common in DFB lasers. This allowspositive feedback of the optical energy at the signal wavelength, suchthat the desired resonance condition is achieved. Unlike the traditionalDFB laser, however, the FIG. 1A embodiment uses not only signal grating12, which is reflective at the signal wavelength, but also pump grating14, which is reflective at the wavelength of the optical energy fromoptical pumping source 15, which is used to pump the optical fiber.

In the embodiment of FIG. 1A, both of the gratings 12, 14 extend alongthe gain portion of the fiber 10, occupying the same longitudinalsection of fiber. The pump grating 14 has a DFB-type structure like thatof the signal grating 12, but it is constructed to be reflective at thepump energy wavelength. The result of this positioning of the twogratings is that, within the gain portion of the fiber, the gratingsprovide a resonant condition for both the signal energy and the pumpenergy. The signal wavelength λ_(s) is reflected within the gain regionby the signal grating 12, while the pump wavelength λ_(p) is reflectedwithin the gain region by the pump grating 14. The reflection of λ_(s)provides the desired resonance condition, as in a conventional fiberlaser. The reflection of λ_(p), meanwhile, contains the pump energywithin the gain portion of the optical fiber 10, where it may beabsorbed by the doped fiber material, allowing the stimulated emissionof optical energy at the signal wavelength λ_(s). This "resonantpumping" of the short fiber laser allows it to develop higher signalpowers than conventional short fiber lasers in which the pump energysimply passes once through the gain medium.

The embodiment of FIG. 1B is also a resonant pumped short fiber laser11, but rather than using a pump grating that extends the length of thegain medium, two pump gratings 14 are used, one on either side of thelaser cavity. The cavity is defined by the location of signal grating12, which is essentially the same as the signal grating 12 of FIG. 1A.The gratings 14 cause pump energy to be reflected back and forth throughthe doped fiber in the region containing the signal grating 12, suchthat the pump energy is absorbed, and spontaneous emission of opticalenergy at the signal wavelength λ_(s) results. This arrangement of thepump gratings is similar to that of a distributed Bragg reflector (DBR)laser in that the pump gratings are positioned to either side of theactive region of the laser.

Shown in FIG. 1C is another embodiment of the short fiber laser 11. Thisembodiment is again similar to a DBR laser in that the signal gratings12 are both positioned outside of the active region of the laser. As inthe FIG. 1B embodiment, the pump gratings 14 are also outside of theactive region, and the resulting structure provides resonance within thelaser cavity at both the signal wavelength A_(s) and at the pumpwavelength λ_(p). The optical signal energy and the pumping energy isreflected back and forth within the cavity, each between its respectiveset of wavelengths-selective gratings. The erbium-doped fiber 10 absorbsthe optical energy at the pump wavelength λ_(p), and spontaneousemission of optical energy at the signal wavelength λ_(s) results ingeneration and amplification of the optical signal, which resonateswithin the cavity established by the signal gratings. Thus, wherein theFIG. 1B embodiment uses a DFB type signal reflection apparatus with aDBR type pump resonator, the FIG. 1C embodiment uses a DBR type signalreflection apparatus with a DBR type pump resonator.

FIG. 1D is yet another embodiment of a resonant-pumped short fiber laseraccording to the present invention. The FIG. 1D embodiment is verysimilar to that of FIG. 1C, using a DBR type signal reflection apparatus12 with a DBR type pump resonator 14. Unlike FIG. 1C, however, theembodiment of FIG. 1D also includes a Q-switch 13. The concept ofQ-switching is well-known in the art, and involves inhibitingoscillation within the resonator cavity until a significant populationinversion has developed, at which time the obstacle to feedback isremoved and the laser quickly develops a high degree of oscillation anda high power, short duration ("giant") pulse is emitted. The Q-switch 13of FIG. 1D may be any of a number of known types of Q-switch, and ispositioned between the signal gratings 12. In this embodiment, if a 1550nm signal wavelength is used, and the fiber is erbium-doped, it ispreferred that the pump wavelength be 1480 nm, that is, the gratings 14in FIG. 1D are constructed to have a peak resonant condition atapproximately 1480 nm. This is because under the Q-switched process, thefiber is very strongly pumped and, under these conditions, the use of a980 nm pump wavelength would result in severe losses due to a two photonup conversion process (excited state absorption--ESA).

In FIG. 1E is shown a short fiber amplifier structure similar to that ofthe short fiber lasers shown in FIGS. 1A-1D. Unlike the fiber laserembodiments, the signal wavelength is not generated within the gainmedium, but is coupled together with the pump wavelength using anoptical coupler such as WDM 17. Pump resonator, established by gratings14, keeps the pump energy oscillating within the gain medium, where itcan be absorbed by the doped fiber. This resonant pumping maintains apopulation inversion in the amplifier gain medium. Thus, the signalenergy is amplified through stimulated emission from the pumped fiber.

FIG. 1F depicts an embodiment of the present invention in which resonantpumping is used to provide population inversion in an amplifiedspontaneous emission (ASE) source. Like the above embodiments, the ASEsource uses doped optical fiber and a pump resonator consisting ofgratings 14. Optical pumping source provides the pump energy whichresonates between the gratings 14, being absorbed by the optical fiber.However, rather than providing a single wavelength signal resonancecavity, as in the fiber laser embodiments, the pumped fiber 10 is simplyallowed to generate a broadband output. A broadband grating 21 islocated between the pump source 15 and the fiber gratings 14 to ensurethat the ASE output (indicated in the figure by the symbol λ_(ASE) isdirected in the direction away from the pump source 15.

In each of the embodiments of FIGS. 1A, 1B 1C and 1D, the wavelengthsused are selected based on the desired signal output and the dopingmaterial used for the fiber 10. For example, a preferred embodiment maybe designed for an optical signal at a wavelength of λ_(s) =1550 nm,which is near the transmission peak of conventional silica-based opticalfiber. In such a case, an erbium-doped optical fiber may be used, andmay be pumped with optical energy at a wavelength of, e.g., λ_(p) ≅980nm. Similarly, an erbium/ytterbium doped fiber could be used, and couldbe pumped with optical energy at a wavelength of, e.g. λ_(p) ≅1060 nm.

Referring to FIGS. 2A-2C, a tunable embodiment of the invention is shownin which two short fiber lasers are arranged in series. In FIG. 2A, twolasers each having the construction of the laser of FIG. 1A are formedby different gratings etched into doped optical fiber 10. Fiber laser11a is constructed in the fiber from signal grating 12a and pump grating14a, while fiber laser 11b is constructed from signal grating 12b andpump grating 14b. The signal gratings 12a, 12b are bothwavelength-selective, each being reflective at a different wavelengthwithin the gain profile of the doped optical fiber. For example, iffiber 10 is doped with erbium/ytterbium, the signal grating 12a may beconstructed to be highly reflective at a wavelength of λ_(s1) =1552 nm,while grating 12b is constructed to be highly reflective at a wavelengthof λs2=1550 nm. Likewise, the pump gratings 14a, 14b are also wavelengthselective and each reflective at a different wavelength within theacceptable range of pump wavelengths for the fiber. For anerbium/ytterbium doped fiber 10, pump grating 14a may be made highlyreflective at a wavelength of λ_(p1) =1062 nm, while pump grating 14b ishighly reflective at a wavelength of λ_(p2) =1060 nm.

The wavelength discrimination of each signal grating is high enough thatoptical signal energy at the wavelength of one grating is notsignificantly affected by the other grating. For example, optical energyat λ_(s1), while being reflected by grating 12a, bypasses grating 12bwith only a small amount of loss due to interaction between the lightand the grating 12b. The same is true for the wavelength discriminationof the pump gratings. That is, optical energy at wavelength λ_(p1)suffers only a relatively small amount of loss due to interaction withthe pump gratings 14b. This high discrimination of the gratings 12a,12b, 14a, 14b allows the overall laser device to have a selectableoutput wavelength.

The input to the laser embodiment of FIG. 2A is a tunable pump energysource 19, which may be tuned to a wavelength of either λ_(p1) orλ_(p2). When the input pump source is tuned to λ_(p1), the pump energyis reflected within the gain medium of short fiber laser 11a by pumpgrating 14a. This reflection keeps most of the pump energy resonatingwithin the cavity of laser 11a, and allows it to be absorbed by thefiber, which then generates optical energy through stimulated emission.The optical energy output within the cavity includes optical energy atthe signal wavelength λ_(s1) which is within the range of outputwavelengths for doped optical fiber 10. Since signal grating 12a ishighly reflective at λ_(s1), a resonance condition at this wavelength isestablished within the cavity of short fiber laser 1a. The grating 12ais selected to allow a certain amount of transmission in the outputdirection of the laser 11a (i.e. toward the right at the orientationshown in FIG. 2A). Thus, laser energy at wavelength λ_(s1) is outputfrom the short fiber laser 11a toward short fiber laser 11b. However,because of the high degree of discrimination of the gratings of bothlasers, the desired λ_(s1) output passes through the laser 11b with onlya small degree of loss due to reflection from grating 12b. Similarly,any λ_(p1) pump energy leaking out of the first short fiber laser 11adoes not significantly interact with gratings 14b of laser 11b, andpumping of the second laser cavity is therefore minimized.

When an output at wavelength λ_(s2) is desired, the wavelength of theinput optical energy is set at λ_(p2). Because of the highdiscrimination of the gratings, very little loss of the λ_(p2) pumpenergy occurs as it passes through fiber laser 11a. However, the highreflectivity of gratings 14b result in the λ_(p2) pump energy resonatingwithin the gain medium of fiber laser 11b. Absorption of the pump energyresults in stimulated emission within the cavity of laser 11b, includingemission of optical energy at wavelength λ_(s2). This energy isreflected within the cavity of laser 11b due to the high reflectivity ofsignal grating 12b, and a resonance condition is established. The signalgrating 12a is selected to allow partial transmission of the λ_(s2)optical energy out of the laser 11b cavity, providing the necessaryoutput coupling of the laser. Thus, the laser output may be selected tobe either λ_(s1) or λ_(s2) by tuning the wavelength of the input pumpenergy.

The embodiments of FIGS. 2B and 2C function in essentially the same wayas the embodiment of FIG. 2A, except for the specific construction ofeach of the fiber lasers 11a, 11b. In FIG. 2B, short fiber lasers 11a,11b each have the same construction as the short fiber laser of FIG. 1B,that is, each has a single signal grating (12a and 12b, respectively)which extends the length of the gain medium, and two pump gratings (14aand 14b, respectively) at the ends of the gain medium. For each laser11a, 11b, the signal grating is partially reflective toward the outputside of the laser (i.e. to the right in the orientation of FIG. 2B) toprovide the output coupling of each laser. As in the embodiment of FIG.2A, the wavelength discrimination of the gratings is high enough thatthere is very little interference with one signal wavelength by the pumpgrating constructed to reflect the other signal wavelength. Similarly,there is very little interference with one pump wavelength by the pumpgratings constructed to reflect the other pump wavelength. By selectingbetween input pump wavelengths λ_(p1), λ_(p2), one may select betweenoutput signal wavelengths λ_(s1), λ_(s2).

The embodiment of FIG. 2C also functions in the same manner as theembodiment of FIG. 2A, except that the short fiber lasers 11a, 11b havea construction like that of the laser shown in FIG. 1C (or alternativelyuse the Q-switched embodiment of FIG. 1D). That is, in each laser 11a,11b, signal gratings (12a and 12b, respectively) are to either side ofthe gain medium, as are two pump gratings (14a and 14b, respectively).When input wavelength λ_(p1) is selected, the gratings 14a reflect thepump energy back and forth across the gain medium of short fiber laser11a, allowing the absorption of pump energy and stimulated emission ofsignal energy. The signal grating 12a to the output side of the laser(i.e. to the right in the orientation of FIG. 2C) has a degree ofpartial reflectivity which allows some of the signal energy to becoupled out of the laser. When the input wavelength selected is λ₂, thepump energy substantially bypasses laser 11a, and is reflected betweenpump gratings 14b of laser 11b, resulting in its absorption and thegeneration of optical energy at wavelength λ_(s2). The signal grating12b to the output side of laser 11b (i.e. to the right in theorientation of FIG. 2C) is partially transmissive to allow outputcoupling of the λ_(s2) laser energy. Thus, as in the embodiments ofFIGS. 2A and 2B, selection between input pump wavelengths λ_(p1), λ_(p2)with source 19 allows one to select between output signal wavelengthsλ_(s1), λ_(s2).

For each of the embodiments of FIGS. 2A-2C, it is preferable that thesignal grating(s) of the laser 11a be constructed to reflect longerwavelengths than the signal grating(s) of laser 11b. This is due to thefact that the interference encountered by shorter wavelengths passingthrough gratings set to reflect higher wavelengths is less than theinterference encountered by longer wavelengths passing through gratingsarranged to reflect shorter wavelengths. Thus, the series arrangement ofshort fiber lasers in this manner should always have lasers withgratings constructed to reflect longer wavelengths preceding those withgratings constructed to reflect shorter wavelengths.

It should also be readily apparent to one skilled in the art that thetunable laser configurations of FIGS. 2A-2C should not be consideredlimited to only two lasers. While only two lasers are shown in each ofthese figures, additional short fiber lasers with different gratings canbe placed in series with lasers 11a, 11b to allow additional tunableoutput wavelengths. Furthermore, the embodiments of FIGS. 2A-2Cincorporate the resonant pumped laser structures shown in FIGS. 1A-1Dbut, in each embodiment, use the same type of structure for both of theseries-linked lasers. Those skilled in the art will recognize that thedifferent laser structures shown in FIGS. 1A-1D can be mixed and matchedto arrive at a desired configuration. For example, a series combinationof the structure of FIG. 1A and the structure of FIG. 1C could be used.

Depicted in FIGS. 3A and 3B are variations of the wavelength selectiveembodiments of FIGS. 2B and 2C in which the output signals for each ofthe lasers 11a, 11b are extracted directly from the lasers themselvesusing wavelength division multiplexers. The arrangement shown in FIG. 3Ais the same as that of FIG. 2B, except that wavelength divisionmultiplexer (WDM) 16a is located between signal grating 12a and theoutput side pump grating 14a of the laser 11a. The WDM 16a divertsoptical energy at a wavelength of λ_(s1) to a desired output location.While WDM 16a is only schematically shown in the figure, the general useof WDMs is well-known in the art. By using WDM 16a as the output pathfor the laser energy of short fiber laser 11a, the λ.sub._(s1) laseroutput does not have to pass through short fiber laser 11b. Thus, anylosses due to interaction with the gratings of laser 11b are avoided.Obviously, the pump energy λ_(p1), which pumps laser 11a, is notdiverted by the WDM 16a as it reflects back and forth through the gainmedium of laser 11a. Likewise, pump energy at the wavelength λ_(p2)passes through the WDM to pump the fiber laser 11b.

In the fiber laser 11b of FIG. 3A, WDM 16b is used to divert the laseroutput at wavelength λ_(s2) to a desired output location. As in laser11a, the pump energy is not diverted, and remains within the lasercavity, reflecting between the gratings 14b. Those skilled in the artwill recognize that the WDM 16b could be omitted if the system had onlythe two lasers 11a, 11b, and there would be little difference in theλ_(s2) signal loss. However, WDM 16b is included in FIG. 11b since theembodiment may also include additional short fiber lasers havingdifferent output wavelengths. If these additional fiber lasers arelocated to the output side of laser 11b (i.e. to the right of laser 11brelative to the orientation of FIG. 3B), WDM 16b prevents the λ_(s2)output signal from having to pass through the signal gratings of thesubsequent lasers, where some losses might occur.

The configuration of FIG. 3B is identical to that of FIG. 2C, exceptthat WDMs 16a, 16b are used in the same manner as shown in FIG. 3A. Thatis, the WDMs 16a, 16b are used in the FIG. 3B embodiment to divert thelaser signal energy individually out of each laser to avoid any losseswhich might occur as it passes through downstream laser components.Again, the WDM 16b is included to allow for the diversion of signalenergy at a wavelength of λ_(s2) should additional short fiber lasers beused. In each of the embodiments of FIGS. 3A and 3B, the arrangementprovided allows for a selectable laser output which is changed bychanging the input pump wavelength, as is described above with regard toFIGS. 2A-2C. A further embodiment can also be used which functions inthe same manner as the embodiments of FIGS. 3A and 3B, but which uses alaser structure like that of FIG. 1B in one stage, while using a laserstructure like that of FIG. 1C in the other stage.

Because the resonant pumping laser systems described herein rely on ahigh degree of wavelength discrimination for performance, accuratecontrol of the light input to the lasers can be very important. This isparticularly true when using light sources that can drift in wavelength.For example, sources such as semiconductor lasers are known to sufferdrift of their output wavelength with changes in time and/ortemperature.

Shown in FIG. 4A is a fiber laser system which uses electronic feedbackto maintain its wavelength stability of the pump source. The source ofpumping energy is a single-wavelength semiconductor pump laserconsisting of semiconductor diode 20 and doped optical fiber 22 (forease of description, optical fibers in FIGS. 4A and 4B are each shownschematically as a line). Such pump lasers are well-known in the art,and provide a single wavelength optical output. However, changes in timeand temperature may cause the pump laser output to drift slightly inwavelength. Since this laser source is to be used as a pump source for aresonant-pumped short fiber laser having reflectors at the peak pumpwavelength, it is important to prevent such drift so that the output ofthe pump laser remains stable at the desired pump wavelength. To dothis, the embodiment of FIG. 4A uses electronic feedback to control thecurrent driving the diode, as described in more detail below. Sincechanges in the diode driver current cause changes in the outputwavelength of the diode, this feedback may be used to combat wavelengthdrift in the pump laser.

The semiconductor diode 20 is current driven, and has a DC power inputprovided by DC voltage source. The power input to the diode is alsomodulated by AC voltage source 36, the amplitude of which is a fraction(e.g. several percent) of the DC voltage from voltage source 36.Inductor 40 and capacitor 42 help prevent any interaction between thetwo sources 36, 38. Thus, the electrical power to the diode 20 (andtherefore the optical power output) is modulated about a particularmagnitude at the frequency of the AC voltage source 36. This modulationresults in an FM signal with two small side bands in addition to thecarrier of the unmodulated pump laser.

The output of doped fiber 22 is coupled into optical isolator 22. Theoptical isolator 22 is a unidirectional optical element that is known inthe art and commercially available. The isolator 22 prevents any opticalenergy from being fed back to the diode and disrupting its operation.From the isolator 22, the pump energy passes to optical coupler 26.Optical coupler 26 is some type of beamsplitter, such as a wavelengthdivision multiplier. It passes most of the optical energy through toshort fiber laser 11, which consists of signal gratings 12 and pumpgratings 14. The fiber laser 11 shown in FIG. 4A is the sameconstruction as that of FIG. 1C. However, the electronic feedbackarrangement shown may be as easily used with the resonant pumped laserarrangements shown in FIGS. 1A and 1B.

The optical energy provided by the semiconductor pump laser is reflectedbetween pump gratings 14 as described in previous embodiments. Thisresonance keeps the pump energy in the gain medium where it is absorbedby the fiber, which then outputs optical energy at the desired signalwavelength by stimulated emission. The combination of gratings 12, 14 tothe input side of the fiber laser 11 also reflects a small amount (e.g.5%) of the pump energy reaching it. When the pump energy input to thefiber laser 11 is at the correct wavelength, the peak resonant conditionfor the gratings is achieved. However, if the input energy is "offresonance" (i.e. the wavelength is different from that for which thegratings 14 are constructed), the optical energy reflected back towardoptical coupler 26 will have an increased intensity. In addition to theincrease in reflectivity, a non-resonant condition will result in adifferent phase shift for the two side bands of the FM signal. Thisphase shift, in turn, results in an FM-to-AM conversion. The magnitudeof the AM signal is a measure of how far the pump laser isoff-resonance. This may then be used to provide the desired feedbackcontrol of the diode 20.

After reflection from the input pump grating 14 of laser 11, thisreflected pump energy returns to the directional coupler, and a portionof it is directed to filter/attenuator 28. Filter/attenuator 28 is anoptional component which may be used to block wavelengths outside of thedesired range of the pump wavelength. This minimizes the relative noisein the reflected pump energy. Filter/attenuator 28 may also simplyreduce the magnitude of the feedback signal to a desirable level. Afterexiting the filter/attenuator 28, the reflected pump energy is input tophotodetector 30, which converts the optical energy into an electricalfeedback signal.

The electrical feedback signal has the characteristics of the reflectedoptical signal, including any intensity change and phase shift. Thissignal is amplified by linear amplifier 32, and is input to mixer 34.Mixer 34 is an electronic frequency mixer, as is know in the art, andcombines the amplified feedback signal with the AC signal from ACvoltage source 36. Because the frequencies of the signals are identicalor nearly identical, the output from mixer 34 is a DC voltage. Thisvoltage is amplified using amplifier 44. The magnitude of the DC voltageis affected by the relative phase of the feedback signal and the ACinput signal. This phase difference is, in turn, affected by thedifference between the resonant wavelength of the pump gratings 14(i.e., the wavelength at which a peak resonance condition is achieved bythe gratings 14) and the wavelength of the optical energy from thediode. If the output wavelength of the diode drifts shorter, therelative phase shift will cause the DC signal output from mixer 34 todecrease in magnitude. This, in turn, causes the wavelength output fromthe diode to increase, moving it back toward the peak resonancecondition of the gratings 14. Similarly, if the output wavelength of thediode becomes longer, the DC signal output from mixer 34 increases inmagnitude. This causes the wavelength of the diode optical output todecrease, again moving it toward the peak resonance condition of thegratings 14.

FIG. 4B shows another feedback arrangement for use with the resonantpumped short fiber lasers disclosed herein. However, rather than usingelectronic feedback, as in the embodiment of FIG. 4A, the FIG. 4Bembodiment uses optical feedback. The optical source of pumping energyis again a fiber laser consisting of semiconductor laser diode 20 anddoped fiber coil 22. The pumping energy is input to optical circulator46. Optical circulator 46 is a device which is known in the art andwhich is commercially available. The optical circulator is aunidirectional optical element which may have multiple inputs andoutputs. The pump energy from fiber coil 22 is input to the circulator,but is output only at the input to short fiber laser 11. Since thecirculator 46 is unidirectional, no optical transmission can pass backthrough the circulator from laser 11 to fiber coil 22. Similarly, anyoptical energy which is output by filter/attenuator 50 (discussedfurther below) is directed only to the coil 22, with no optical energybeing allowed to go the opposite direction.

The optical pump energy exiting circulator 46 is introduced to the fiberlaser 11, and is reflected between the pump gratings 14, allowing it tobe absorbed by the doped fiber of the laser. Again, the fiber laser 11is the same construction as the laser shown in FIG. 1C but, for thepurposes of this feedback embodiment, could just as well be either ofthe embodiments shown in FIGS. 1A and 1B. For pump energy which is "onresonance" (i.e. which is at the peak reflective wavelength of thegratings 14) there is virtually no loss of optical pump energy enteringthe fiber laser. However, the further off resonance the wavelength ofthe input pump energy drifts, the more of it gets reflected back towardthe circulator. This, in turn, reduces the amount of pump energyentering the fiber laser cavity.

The pump grating 14 toward the output side of the laser 11 (i.e. towardthe right in the orientation of FIG. 4B) allows a small fraction of"on-resonance" pump energy to be transmitted past it. This pump energy,along with the signal energy coupled past the partially transmissiveoutput signal grating 12, is directed to optical coupler 48. Opticalcoupler 48 is preferably a wavelength division multiplexer, and directsoptical energy in the wavelength range of the pump energy towardfilter/attenuator 50, while directing energy in the wavelength range ofthe laser 11 signal output, λ_(s), toward an output port of the laser.The pump energy reaching filter/attenuator 50 may be bandpass filteredto eliminate excess noise in the returning pump signal, or may beattenuated to prevent a feedback signal of too high a magnitude. Thefilter/attenuator 50 is a device known in the art, and is optionaldepending on the specific design specifications of the system.

The pump energy output from the wavelength division multiplexer 48constitutes an optical feedback signal, and is used to control theoutput of the fiber laser source consisting of diode 20 and dopedoptical fiber 22. After passing through the filter/attenuator 50, thefeedback signal passes through circulator 46 toward the source laser.The interaction of the feedback signal with the laser causes it toresonate at the wavelength of the feedback signal. That is, the laser"locks on" to the wavelength of the feedback signal. Since thewavelength of the feedback signal is the "on-resonance" wavelength ofthe pump gratings 14 of short fiber laser 11, this forces the outputfrom source laser 20, 22 to match the peak resonance wavelength of thepump gratings 14, and thus prevents the wavelength drift of the pumpsource.

Shown in FIG. 5A is an arrangement for pumping a plurality of shortfiber lasers simultaneously. An optical pumping source, such assemiconductor laser diode 60, provides optical energy at a desiredwavelength. For a fiber medium doped with erbium, laser source 60, whichmay be a semiconductor laser diode, would typically have a wavelength ofapproximately 980 nm. The output of diode 60 is input to an opticalsplitter 62. Splitter 62 is an optical element of known design, such asa fused fiber directional coupler, that divides the input signal fromdiode 60 into a plurality of outputs, each of the outputs receiving thesame optical power. That is, the splitter provides a equal division ofthe input signal optical power to each of its multiple outputs.

Each outputs of splitter 62 is directed to one of a number of shortfiber lasers. Each fiber laser includes a highly reflective grating64a-64h (generally referred to as 64) and a partially reflective grating66a-66h (generally referred to as 66) that serves as the laser outputcoupler. In the preferred embodiment, the gratings 64, 66 of each of thefiber lasers are constructed to resonate at a different wavelength thanthat of the other lasers. In this way, the arrangement provides aplurality of different output signals (eight in the embodiment shown),each of which has a different wavelength (the different outputwavelengths are indicated in FIG. 5A as λ₁ -λ₈). Furthermore, each ofthe laser signals has the same output power as the others, therebyproviding a user with a good set of optical signals for multiplexedcommunications applications.

In FIG. 5B is depicted an embodiment in which the outputs from aplurality of short fiber lasers are all coupled into a single fiber. Asin FIG. 5A, the embodiment of FIG. 5B uses a plurality of short fiberlasers, each of which resonates at a different wavelength (in thefigure, the different wavelengths are indicated as λ₁ -λ₈). However, inthe embodiment of FIG. 5B, the partially reflective gratings 66a-66h,which function as the laser output couplers, are to the side of thelaser facing optical splitter 62, while the highly reflective gratings64a-64h are to the side of the laser away from splitter 62. The lasersare pumped by an optical source, such as semiconductor laser diode 60,which is coupled into the system output fiber via WDM 68. The opticalpower of the pumping signal is split between the different fiber lasersby splitter 62, such that the pumping power to each of the lasers isapproximately equal. As the different fiber lasers resonate at theirdifferent respective wavelengths, the output coupling gratings 66 of thelasers direct the laser outputs back through the splitter 62 and intothe output fiber. The wavelengths output from the lasers aresignificantly different than the pumping wavelength and are directedtogether by WDM 68 to output port 70.

The feedback arrangements of FIGS. 4A and 4B may also be applied to themultiple short fiber laser embodiments of FIGS. 5A and 5B. Multiplelaser embodiments using electronic feedback are shown in FIGS. 6A and6B, FIG. 6A being essentially a combination of the embodiments of FIG.4A and FIG. 5A, while FIG. 6A is essentially a combination of theembodiments of FIG. 4A and FIG. 5B. In each of the FIG. 6A and FIG. 6Bembodiments, it is only necessary to return optical feedback energy fromone of the short fiber laser branches, since the resonator modes of thedifferent lasers are close in wavelength. Preferably, the laser fromwhich optical energy is fed back is one in the middle of the wavelengthrange of the different lasers.

In each of FIGS. 6A and 6B, reference numerals are used which matchthose of earlier figure. In each of these figures, the lasers are alsoresonant pumped, having pump reflector gratings 63a-63h and 65-65h inaddition to signal reflector gratings 64a-64h and 66a-66h. In FIG. 6A,the output wavelength of pumping source 60 is controlled usingelectronic feedback in the same manner as described above with regard toFIG. 4A. The optical reflection signal is taken from the fiber laserhaving gratings 63a, 64a, 65a, 66a. This arrangement of lasers is thenadequately pumped to produce the desired multiple outputs λ₁ -λ₈. InFIG. 6B, the same type of electronic feedback control is used, and theoptical feedback signal is again taken from the laser resonator definedby gratings 63a, 64a, 65a, 66a. This allows stable pumping in thedesired range, and provides the desired combined output of wavelengthsλ₁ -λ₈.

The embodiments of FIGS. 7A and 7B are similar to those of FIGS. 6A and6B, except that they use an optical feedback arrangement like that shownin FIG. 4B, rather than the electronic feedback version. The referencenumerals in FIGS. 7A and 7B match those of earlier figures, whereappropriate. As in the embodiments of FIGS. 6A and 6B, the embodimentsof FIGS. 7A and 7B use resonant pumping with pump gratings 63a-63h and65a-65h. In FIG. 7A, some of the output from the laser defined bygratings 63a, 64a, 65a, 66a is returned to the fiber laser source viaWDM 48, attenuator 50 and optical circulator 46. This stabilizes theoutput wavelength of the optical source, so that all of the lasers areadequately pumped, and produce the desired multiple outputs havingwavelengths λ₁ -λ₈, respectively. In FIG. 7B, part of the output fromthe laser defined by gratings 63a, 64a, 65a, 66a is returned to thesource via WDM 48, attenuator 50 and optical circulator 46 andstabilizes the output wavelength of the source. This allows the pumpingwavelength to be sufficient for all of the lasers, the outputs of whichare combined to create the output at port 70 consisting of wavelengthsλ₁ -λ₈.

While the invention has been shown and described with regard to apreferred embodiment thereof, it will be recognized by those skilled inthe art that various changes in form and detail may be made hereinwithout departing from the spirit and scope of the invention as definedby the appended claims. For example, while the concepts above have beendiscussed in the context of optical fiber waveguides, they may be easilyextended to waveguide resonators in other media including, but notlimited to, planar silica-based (SiO₂) waveguides that are formed onsilicon substrates, or lithium niobate (LiNbO₃) passive or activewaveguides formed on LiNbO₃ substrates.

What is claimed is:
 1. An optical source for generating a predeterminedoptical signal comprising a predetermined signal wavelength, the sourcecomprising:an optical gain medium; a pump resonator that establishes aresonant cavity for optical energy at a pump wavelength suitable foroptically pumping the gain medium, the resonant cavity including thegain medium; and an optical pump source external to the resonant cavitythat directs optical energy at the pump wavelength into the resonantcavity.
 2. An optical source according to claim 1 further comprising asignal reflection apparatus that reflects light at the signal wavelengthwithin the gain medium to partially retain light at the signalwavelength within the gain medium.
 3. An optical source according toclaim 2 wherein the signal reflection apparatus comprises a periodicgrating that is built into a pumped part of the gain medium.
 4. Anoptical source according to claim 3 wherein the periodic grating of thesignal reflection apparatus includes a structural characteristic thatcauses a 1/4-wavelength phase shift in light at the signal wavelength.5. An optical source according to claim 3 wherein the pump resonatorcomprises a periodic grating that is built into a part of the gainmedium occupied by the periodic grating of the signal reflectionapparatus.
 6. An optical source according to claim 5 wherein theperiodic grating of the pump resonator includes a structuralcharacteristic that causes a 1/4-wavelength phase shift in light at thepump wavelength.
 7. An optical source according to claim 3 wherein thepump resonator comprises a first periodic grating that is located in thegain medium to a first side of the gain medium, and a second periodicgrating located in the gain medium to a second side of the gain medium.8. An optical source according to claim 2 wherein the signal reflectionapparatus comprises a first periodic grating located in the gain mediumrelatively close to the output side of the gain medium, and a secondperiodic grating located in the gain medium relatively close to theinput side of the gain medium.
 9. An optical source according to claim 8wherein the pump resonator comprises a first periodic grating that islocated in the gain medium to a first side of the gain medium of thesignal reflection apparatus, and a second periodic grating located inthe gain medium to a second side of the gain medium of the signalreflection apparatus.
 10. An optical source according to claim 9 furthercomprising a Q-switch.
 11. An optical source according to claim 2wherein the pump resonator at least partially overlaps the signalreflection apparatus in the gain medium.
 12. An optical source accordingto claim 1 wherein the gain medium is a silica-based fiber doped witherbium, and the predetermined signal wavelength is approximately 1550nm.
 13. An optical source according to claim 12 wherein the pumpwavelength is approximately 980 nm.
 14. An optical source according toclaim 1 wherein the gain medium is a silica-based fiber doped witherbium/ytterbium, the predetermined signal wavelength is approximately1550 nm, and the pump wavelength is in the wavelength range ofapproximately 980 nm-1060 nm.
 15. An optical source according to claim 1wherein the optical source comprises an optical amplifier, and anoptical signal comprising the signal wavelength is input to, andamplified within, the gain medium.
 16. An optical source according toclaim 1 further comprising a broadband signal reflector that reflects aband of wavelengths, including the predetermined signal wavelength, thatresult from amplified spontaneous emission within the gain medium,toward an output of the optical source.
 17. An optical source accordingto claim 1 further comprising an optical waveguide within which theoptical gain medium is located.
 18. An optical source apparatus forgenerating an optical signal at one of a plurality of different signalwavelengths, the source comprising:a first optical gain medium; a firstsignal reflection apparatus that reflects light at a first one of thesignal wavelengths within the first gain medium so as to retain light atsaid first signal wavelengths within the first gain medium and providean amplified signal output from the first gain medium at said firstsignal wavelength; a second optical gain medium; a second signalreflection apparatus that reflects light at a second one of the signalwavelengths within the second gain medium so as to retain light at saidsecond signal wavelength within the second gain medium and provide anamplified signal output from the second gain medium at said secondsignal wavelength; an optical pump source with a selectable outputwavelength that is capable of coupling light at a first pump wavelengthinto the first gain medium and capable of coupling light at a secondpump wavelength into the second gain medium; a first pump resonator thatestablishes a first resonant cavity for optical energy at the first pumpwavelength, the first resonant cavity including the first gain medium;and a second pump resonator that establishes a second resonant cavityfor optical energy at the second pump wavelength, the second resonantcavity including the second gain medium.
 19. An optical source apparatusaccording to claim 18 wherein the first signal reflection apparatuscomprises a periodic grating that is built into a pumped part of thefirst gain medium, the periodic grating comprising a structuralcharacteristic that causes a 1/4-wavelength phase shift in light at thefirst signal wavelength.
 20. An optical source apparatus according toclaim 19 wherein the second signal reflection apparatus comprises aperiodic grating that is built into a pumped part of the second gainmedium, the periodic grating of the second signal reflection apparatuscomprising a structural characteristic that causes a 1/4-wavelengthphase shift in light at the second signal wavelength.
 21. An opticalsource apparatus according to claim 20 wherein:the first pump resonatorcomprises a periodic grating that is built into a part of the first gainmedium occupied by the first signal reflection apparatus and theperiodic grating of the first pump resonator includes a structuralcharacteristic that causes a 1/4-wavelength phase shift in light at thefirst pump wavelength; and wherein the second pump resonator comprises aperiodic grating that is built into a part of the second gain mediumoccupied by the second signal reflection apparatus and the periodicgrating of the second pump resonator includes a structuralcharacteristic that causes a 1/4-wavelength phase shift in light at thesecond pump wavelength.
 22. An optical source apparatus according toclaim 19 wherein the first pump resonator comprises an input periodicgrating located to an input side of the first gain medium, and an outputperiodic grating located to an output side of the first gain medium. 23.An optical source apparatus according to claim 22 further comprising awavelength selective element located between the first signal reflectionapparatus and the output periodic grating of the first pump resonator,the wavelength selective element coupling light at the first signalwavelength out of the first gain medium to a first wavelength outputport.
 24. An optical source apparatus according to claim 22 wherein:thesecond signal reflection apparatus comprises a periodic grating that isbuilt into a pumped part of the second gain medium, the periodic gratingof the second signal apparatus comprising a structural characteristicthat causes a 1/4-wavelength phase shift in light at the second signalwavelength; and the second pump resonator comprises an input periodicgrating located to an input side of the second gain medium, and anoutput periodic grating located to an output side of the second gainmedium.
 25. An optical source apparatus according to claim 24 furthercomprising:a first wavelength selective element located between thefirst signal reflection apparatus and the output periodic grating of thefirst pump resonator, the first wavelength selective element couplinglight at the first signal wavelength out of the first gain medium to afirst wavelength output port; and a second wavelength selective elementlocated between the second signal reflection apparatus and the outputperiodic grating of the second pump resonator, the second wavelengthselective element coupling light at the second signal wavelength out ofthe second gain medium to a second wavelength output port.
 26. Anoptical source apparatus according to claim 18 wherein the first signalreflection apparatus comprises an input periodic grating located to aninput side of the first gain medium, and an output periodic gratinglocated to an output side of the first gain medium.
 27. An opticalsource apparatus according to claim 26 wherein the first pump resonatorcomprises an input periodic grating located to an input side of thefirst gain medium, and an output periodic grating located to an outputside of the first gain medium.
 28. An optical source apparatus accordingto claim 27 further comprising a wavelength selective element locatedbetween the output periodic grating of the first signal reflectionapparatus and the output periodic grating of the first pump resonator,the wavelength selective element coupling light at the first signalwavelength out of the first gain medium to a first wavelength outputport.
 29. An optical source apparatus according to claim 26 wherein thesecond signal reflection apparatus comprises an input periodic gratinglocated to an input side of the second gain medium, and an outputperiodic grating located to an output side of the second gain medium.30. An optical source apparatus according to claim 29 wherein:the firstpump resonator comprises an input periodic grating located to an inputside of the first gain medium, and an output periodic grating located toan output side of the first gain medium; and the second pump resonatorcomprises an input periodic grating located to an input side of thesecond gain medium, and an output periodic grating located to an outputside of the second gain medium.
 31. An optical source apparatusaccording to claim 30 further comprising a Q-switch located in the firstgain medium.
 32. An optical source apparatus according to claim 30further comprising:a first wavelength selective element located betweenthe output periodic grating of the first signal reflection apparatus andthe output periodic grating of the first pump resonator, the firstwavelength selective element coupling light at the first signalwavelength out of the first gain medium to a first wavelength outputport; and a second wavelength selective element located between theoutput periodic grating of the second signal reflection apparatus andthe output periodic grating of the second pump resonator, the secondwavelength selective element coupling light at the second signalwavelength out of the second gain medium to a second wavelength outputport.
 33. An optical source apparatus according to claim 18 furthercomprising an optical waveguide through which the optical signal isdirected.
 34. An optical amplifier for an optical signal at apredetermined signal wavelength, the amplifier comprising:an opticalgain medium; a pump resonator that establishes a resonant cavity foroptical energy at a pump wavelength suitable for optically pumping thegain medium, the resonant cavity including the gain medium; an opticalpump source external to the resonant cavity that directs optical energyat the pump wavelength into the resonant cavity; and an optical couplerthat couples the optical signal into the gain medium such that it isamplified by stimulated emission within the pumped gain medium.
 35. Anoptical amplifier according to claim 34 wherein the pump resonatorcomprises a first periodic grating that is located in the gain medium toa first side of the gain medium, and a second periodic grating locatedin the gain medium to a second side of the gain medium.
 36. An opticalamplifier according to claim 34 wherein the gain medium is asilica-based fiber doped with erbium, and the predetermined signalwavelength is approximately 1550 nm.
 37. An optical amplifier accordingto claim 36 wherein the pump wavelength is approximately 980 nm.
 38. Anoptical amplifier according to claim 34 wherein the gain medium is asilica-based fiber doped with erbium/ytterbium, the predetermined signalwavelength is approximately 1550 nm, and the pump wavelength is in thewavelength range of approximately 980 nm-1060 nm.
 39. An opticalamplifier according to claim 34 wherein the optical coupler comprises awavelength division multiplexer.
 40. An optical amplifier according toclaim 34 further comprising a waveguide within which the optical gainmedium is located.
 41. A method of generating an optical signalcomprising a predetermined signal wavelength, the methodcomprising:providing an optical gain medium; pumping the gain mediumwith a pump resonator that establishes a resonant cavity for opticalenergy at a pump wavelength suitable for optically pumping the gainmedium, the resonant cavity including the gain medium; and coupling intothe gain medium light at a pump wavelength from an optical pump sourceexternal to the resonant cavity.
 42. A method according to claim 41further comprising locating a signal reflection apparatus in the gainmedium that reflects light at the signal wavelength to retain it withinthe gain medium, and provides an amplified optical signal output at thesignal wavelength.
 43. A method according to claim 42 wherein locating asignal reflection apparatus in the gain medium comprises locating aperiodic signal grating in the gain medium.
 44. A method according toclaim 43 wherein locating a periodic signal grating in the gain mediumcomprises locating a periodic signal grating in the gain medium thatincludes a structural characteristic that causes a 1/4-wavelength phaseshift in light at the signal wavelength.
 45. A method according to claim43 wherein pumping the gain medium with a pump resonator compriseslocating a periodic pump grating in a part of the gain medium occupiedby the periodic signal grating.
 46. A method according to claim 45wherein locating a periodic pump grating in the gain medium compriseslocating a periodic pump grating in the gain medium that includes astructural characteristic that causes a 1/4-wavelength phase shift inlight at the pump wavelength.
 47. A method according to claim 45 whereinpumping the gain medium with a pump resonator comprises locating a firstperiodic grating in the gain medium to a first side of the gain mediumand locating a second periodic grating in the gain medium to a secondside of the gain medium.
 48. A method according to claim 42 whereinlocating a signal reflection apparatus in the gain medium compriseslocating a first periodic signal grating relatively close to the outputside of the gain medium and a second periodic signal grating relativelyclose to the input side of the gain medium.
 49. A method according toclaim 48 further comprising Q-switching the portion of the gain mediumbetween the first periodic signal grating and the second periodic signalgrating.
 50. A method according to claim 48 wherein pumping the gainmedium with a pump resonator comprises locating a first periodic pumpgrating between the first periodic signal grating and the output side ofthe gain medium, and locating a second periodic pump grating between thesecond periodic signal grating and the input side of the gain medium.51. A method according to claim 41 further comprising amplifying, withinthe gain medium, an optical signal that is input to the gain medium. 52.A method according to claim 41 further comprising providing a broadbandsignal reflector that reflects a band of wavelengths, including thepredetermined signal wavelength, that result from amplified spontaneousemission within the gain medium, in a direction along which the opticalsignal is ultimately directed.
 53. A method according to claim 41wherein providing an optical gain medium comprises providing an opticalgain medium located in an optical waveguide.
 54. A method of generatingan optical signal at one of a plurality of different signal wavelengths,the method comprising:providing a first gain medium; locating a firstsignal reflection apparatus in the first gain medium that reflects lightat a first one of said signal wavelengths and retains it within thefirst gain medium, and provides an amplified optical signal output atthe first signal wavelength; providing a second gain medium; locating asecond signal reflection apparatus in the second gain medium thatreflects light at a second one of said signal wavelengths and retains itwithin the second gain medium, and provides an amplified optical signaloutput at the second signal wavelength; providing an optical pumpingapparatus capable of coupling a first pump output wavelength into thefirst gain medium and capable of coupling a second pump wavelength intothe second gain medium, the first pump wavelength being in a wavelengthrange sufficient to optically pump the first gain medium and the secondpump wavelength being in a second wavelength range sufficient tooptically pump the second gain medium; pumping the first gain mediumwith a first pump resonator that establishes a first resonant cavity foroptical energy at the first pump wavelength, the first resonant cavityincluding the first gain medium; and pumping the second gain medium witha second pump resonator that establishes a second resonant cavity foroptical energy at the second pump wavelength, the second resonant cavityincluding the second gain medium.
 55. A method according to claim 54wherein locating a first signal reflection apparatus in the first gainmedium comprises locating, in the first gain medium, a periodic gratingthat is built into a pumped part of the first gain medium and whichcomprises a structural characteristic that causes a 1/4-wavelength shiftin light at the first wavelength.
 56. A method according to claim 55wherein locating a second signal reflection apparatus in the second gainmedium comprises locating, in the second gain medium, a periodic gratingthat is built into a pumped part of the second gain medium and thatcomprises a structural characteristic that causes a 1/4-wavelength shiftin light at the second wavelength.
 57. A method according to claim 56wherein:pumping the first gain medium comprises locating, in the firstgain medium, a periodic grating that is built into a part of the firstgain medium occupied by the first signal reflection apparatus and thatincludes a structural characteristic that causes a 1/4-wavelength shiftin light at the first pump wavelength; and pumping the second gainmedium comprises locating, in the second gain medium, a periodic gratingthat is built into a part of the second gain medium occupied by thesecond signal reflection apparatus and that includes a structuralcharacteristic that causes a 1/4-wavelength shift in light at the secondpump wavelength.
 58. A method according to claim 55 wherein pumping thefirst gain medium comprises locating a first periodic pump gratingrelatively close to the output side of the first gain medium and asecond periodic pump grating relatively close to the input side of thefirst gain medium.
 59. A method according to claim 58 further comprisingcoupling light at the first signal wavelength out of the first gainmedium to a first wavelength output port with a wavelength selectiveelement located between the first signal reflection apparatus and theoutput periodic grating of the first pump resonator.
 60. A methodaccording to claim 58 wherein:locating a second signal reflectionapparatus in the second gain medium comprises locating, in the secondgain medium, a periodic grating that is built into a pumped part of thesecond gain medium and which comprises a structural characteristic thatcauses a 1/4-wavelength shift in light at the second wavelength; andpumping the second gain medium comprises locating, in the second gainmedium, an input periodic pump grating between the second signalreflection apparatus and an input side of the second gain medium, and anoutput periodic pump grating between the second signal reflectionapparatus and an output side of the second gain medium.
 61. A methodaccording to claim 60 further comprising:locating a first wavelengthselective element between the first signal reflection apparatus and theoutput periodic grating of the first pump resonator, the firstwavelength selective element coupling light at the first signalwavelength out of the first gain medium to a first wavelength outputport; and locating a second wavelength selective element between thesecond signal reflection apparatus and the output periodic grating ofthe second pump resonator, the second wavelength selective elementcoupling light at the second signal wavelength out of the second gainmedium to a second wavelength output port.
 62. A method according toclaim 54 wherein locating a first signal reflection apparatus in thefirst gain medium comprises locating an input periodic grating to aninput side of the first gain medium and an output periodic grating tothe output side of the first gain medium.
 63. A method according toclaim 62 wherein pumping the first gain medium comprises locating afirst periodic pump grating relatively close to the output side of thefirst gain medium and a second periodic pump grating relatively close tothe input side of the first gain medium.
 64. A method according to claim63 further comprising Q-switching at least one of the first gain mediumand the second gain medium.
 65. A method according to claim 64 furthercomprising coupling light at the first signal wavelength out of thefirst gain medium to a first wavelength output port with a wavelengthselective element located between the first signal reflection apparatusand the output periodic grating of the first pump resonator.
 66. Amethod according to claim 63 wherein locating a second signal reflectionapparatus in the second gain medium comprises locating an input periodicgrating to an input side of the second gain medium, and locating anoutput periodic grating to an output side of the second gain medium. 67.A method according to claim 66 wherein:pumping the first gain mediumcomprises locating an input periodic pump grating relatively close tothe input side of the first gain medium and an output periodic pumpgrating relatively close to the output side of the first gain medium;and pumping the second gain medium comprises locating an input periodicpump grating relatively close to the input side of the second gainmedium and an output periodic pump grating relatively close to theoutput side of the second gain medium.
 68. A method according to claim67 further comprising:coupling light at the first signal wavelength outof the first gain medium to a first wavelength output port with awavelength selective element located between the first signal reflectionapparatus and the output periodic grating of the first pump resonator;and coupling light at the second signal wavelength out of the secondgain medium to a second wavelength output port with a wavelengthselective element located between the second signal reflection apparatusand the output periodic grating of the second pump resonator.
 69. Amethod according to claim 54 wherein providing a first gain mediumcomprises providing a first gain medium located in an optical waveguide.70. A method of amplifying an optical signal comprising a predeterminedsignal wavelength, the method comprising:providing an optical gainmedium; pumping the gain medium with a pump resonator that establishes aresonant cavity for optical energy at a pump wavelength suitable foroptically pumping the gain medium, the resonant cavity including thegain medium; coupling into the gain medium light at a pump wavelengthfrom an optical pump source external to the resonant cavity; andcoupling the optical signal into the gain medium such that it isamplified by stimulated emission within the pumped gain medium.
 71. Amethod according to claim 70 wherein the pumping the gain mediumcomprises locating a first periodic grating in the gain medium to afirst side of the gain medium, and a second periodic grating located inthe gain medium to a second side of the gain medium.
 72. A methodaccording to claim 70 wherein coupling the optical signal into the gainmedium comprises coupling the optical signal into the gain medium with awavelength division multiplexer.
 73. A method according to claim 70wherein providing an optical gain medium comprises providing an opticalgain medium located in an optical waveguide.